The present invention relates generally to oscillators, and more specifically the invention pertains to optical current controlled laser diode oscillators with optoelectronic feedback.
Self-sustained pulsation (SSP) is a dynamic process in laser diodes. It has been extensively studied both experimentally and theoretically. The majority of these previous experiments have been conducted in 780 nm laser diodes. SSP was first considered to be a nuisance and there were efforts aimed at reducing or eliminating SSP. It was realized later on that SSP laser diodes have low relative intensity noise (RIN) because of short coherence length which reduced the effects of optical feedback into the laser diodes. SSP laser diodes have also been proposed for use in all-optical clock recovery.
The present invention relates generally to oscillators, and more specifically the invention pertains to optical current controlled oscillators with optoelectronic feedback. An optical current controlled oscillator is an optical device that produces tunable microwave oscillations (subcarriers) which are already on an optical carrier.
The present invention exploits a nonlinear dynamic in semiconductor laser diodes, namely, transient self-intensity pulsation (TSP). Such semiconductor laser diodes when biased at other current levels exhibit self-sustained pulsation. Self-sustained pulsation (SSP) is a well-known dynamic process in laser diodes. It has been extensively studied both experimentally and theoretically. The majority of these previous experiments have been conducted in 780 nm semiconductor laser diodes. SSP was first considered to be a nuisance and there were efforts aimed at reducing or eliminating SSP. It was realized later on that SSP laser diodes have low relative intensity noise (RIN) because of short coherence length which reduced the effects of optical feedback into the laser diodes. SSP laser diodes have also been proposed for use in all-optical clock recovery. SSP laser diodes can be viewed as an optical current controlled oscillator.
SSP is not the only intensity oscillation, or intensity self-pulsation, dynamic in laser diodes. Self-pulsations in the semiconductor laser diodes fall into two categories, SSP and TSP. In many semiconductor laser diodes, intensity pulsations are predominantly transient in nature. SSP only occurs within a very small range of bias current. For example, a particular semiconductor laser diode exhibited self pulsation for bias currents between 17.0 and 32.0 mA with pulsation frequencies ranging from 3.5 GHz to 6.5 GHz. However, self-sustained pulsation only occurs for bias current values near 20 mA. Self-pulsation for other bias currents were transient, in which the intensity pulsation occurred as soon as the laser diode was turned on or turned to a different bias current. Transient self-pulsations typically only lasted for 1 to 3 minutes. When the bias current is above 20 mA, the frequency of TSP is higher than that of SSP. The present invention exploits TSP and therefore leads to optical current-controlled oscillators with frequencies higher than those which exploit SSP.